[firebase_storage] Expose server error information for "[firebase_storage/unknown] An unknown error occurred, please check the server response."

[gunghoiguana]

We encounter the error [firebase_storage/unknown] An unknown error occurred, please check the server response. intermittently in production (when calling .putFile, on both Android and iOS), but have never replicated it in development.

Right now the FirebaseException does not include any information about the server response or any underlying network error, so it is not possible for us to "check the server response" on our user's devices. (At least, not that I've figured out yet)

Naturally we handle the error as well as we can on the client, but it would be very useful for our development to include the contents of the server response in the FirebaseException, so that we can understand the underlying cause.